Plasma Basics

What is plasma?
  • An ionized gas consisting of ions (mainly positive ions) and free electrons in proportions
  • Electrons are moving much faster than ions, due to mass difference
  • Quasi-zero overall electric charges
  • The 4th state of matter
  • Important parameters about plasma: plasma density, plasma potential, ion energy
How to generate plasma?

Parallel plasma tool as an example. Source

Use parallel plate plasma as an example, voltage applies to the two conductive plates (Anode +, Cathode -) with distance of d. Cathode is located at x=0; anode is at x=d. The following processes are occurring:

  • There are trace amount of electrons and ions in the space even at low pressure without applied electrical field.
  • Under applied electrical field, electrons travel between cathode and anode, colliding with working gas atoms to generate more ions and electrons.
  • Working gas ions hit cathode and release more secondary electrons
Electron density 
Electron Energy Distribution Function (EEDF), source 1 and source 2 (Ar ionization energy)

As shown in above electron energy distribution curves, with increased electron energy, more electrons have energy of 15.8eV, a threshold to ionize Ar atoms. Argon is a most commonly used working gas in sputtering industry.

Electron density needed in different types of sputtering:

  • 1010 #/cm3 for DC glow discharge
    • 1011 #/cm3 for RF parallel plates
    • 1012 #/cm3 for Magnetron plasmas
    • 1013 #/cm3 for ICP plasmas
Breakdown voltage

Two important coefficients are the following.

  • 1st Townscend coefficient α, defined as ionization numbers per electron travel distance
  • 2nd townscend coefficient g, defined as the mean number of generated secondary electrons per ion impact.

Breakdown can be calculated as the following; detailed calculation can be found in the listed source.

Break-down voltage calculation (source)

Paschen’s law

Paschen’s law (Source) shows breakdown voltage as a function of pd, p is pressure in plasma chamber; d is the distance between cathode and anode.

Self-sustaining plasma

Under electrical field, electron energy Ee, must become greater than the ionization energy of the gas atoms, Ei, between the plates, which is nesscary for the ionization of working gas atoms.

To achieve self-sustaining plasma, one electron, traveling from cathode to anode, must have at least one collision, or αd>=1. At αd=1, the break-down voltage is 

Source

Therefore, if three conditions (enough electron density, enough enough to ionize working gas, and αd>=1, self-sustaining plasma can be achieved as shown in following example; different working gas shows different plasma color.

source